Multiple systems use a vector-valued sensor such as a magnetometer, also known as a compass, to measure a strength and direction of the Earth's magnetic field, or an accelerometer to measure acceleration forces applied to a sensor body such as a constant force of gravity and dynamic forces that can be caused by moving or vibrating the sensor body.
Systems that generally measure the strength and direction of the Earth's magnetic field and use results of these measurements as a critical part of providing their core functionality include 9 DOF Inertial Measurement Units (IMU), Attitude and Heading Reference Systems (AHRS), Compassing Systems, and Inertial Navigation Systems (INS).
For accurate results to be produced, each system that uses magnetic sensing elements must be calibrated to account for distortions that affect the sensed magnetic field. These magnetic distortions are generally considered to be one of two types: hard-iron distortions or soft-iron distortions. Hard-iron distortions are caused by objects that produce a magnetic field such as a nearby current carrying conductor or a permanent magnet in a nearby component such as a speaker or microphone. When the magnetic material is part of the system and physically attached to the same reference frame as the sensor, a permanent bias in the sensor output is created. Soft-iron distortions are caused by objects that, by being present, passively distort the reference magnetic field. The presence of these soft-iron objects causes the Earth's magnetic field to be stretched, bent, or otherwise distorted. Soft-iron effects vary depending upon which direction the Earth's magnetic field is oriented relative to the sensor body itself. Soft-iron distortion is often caused by the presence of nearby ferrous metal objects and structures made of materials such as iron, steel or nickel. Hard-iron distortions will generally have a much larger impact upon the total error than soft-iron effects.
Methods exist for performing a calibration process which is used to determine the nature of these error effects within the system and compute parameters to be used in a corrective step as the sensor operates. In this way, a sensor system can be calibrated to correct for both hard-iron and soft-iron effects. Since such systems are often not located within either fixed locations or unchanging environments, such systems are often subject to external effects that can cause changes to the magnetic domain in which the sensor is situated. The system may move through a magnetic environment in which the Earth's magnetic field is distorted or an external object which causes a distortion may move into the sensor's environment. In this manner, the sensor, having been calibrated for the original magnetic environment, will experience erroneous sensor readings due to the altered magnetic environment.
Movement or vibration of an accelerometer sensor body can cause accelerometer perturbations, which can affect accuracy of accelerometer readings such as measurement of a gravitational force or down vector. Parameters can be computed for use in correcting accelerometer measurements and the accelerometer operates. However, if the movement of the accelerometer sensor body changes, accelerometer readings can become distorted due to accelerometer perturbations.